1. Field
Embodiments of the present invention generally relate to the field of control circuits for pump motors. More specifically, embodiments relate to liquid level control circuits which automatically maintain the liquid level within a predetermined range.
2. Background
In sump and water tanks, for example, the liquid level should be maintained within a predetermined range for proper functioning of the tank. Many prior art devices automatically control the liquid level within the tank by activating a pump when the liquid rises above a first predetermined level and deactivating the pump when the liquid level falls below a second predetermined level. Some conventional devices use mechanical or moving parts such as mechanical switches operated by rubber diaphragms, springs, rods, floats, or balls, all of which may tend to wear out or malfunction over time.
Other conventional devices use electrical or optical probes positioned within the tank to determine the liquid level and control the pump accordingly. For example, self-heating thermistors or conductivity probes may be used. However, such conventional systems using probes may be sensitive to humidity, moisture, changing temperatures, and varying voltage levels in the sensing circuit, all of which may produce erroneous results and subject the probes to wear. Also, contamination of the probes may adversely effect their performance. The probes and their associated circuitry may be adjusted to improve performance, but making the adjustments may be inconvenient and expensive.
Employing capacitive sensors for liquid level control provides advantages including the prevention of triggering from transient water imbalances, such as splashes or waves, by precisely defining the required charging time of the capacitive sensors. However, build-up of certain materials, especially dielectric materials, on a capacitive sensor can cause the sensor to detect a false positive. As a result, the pump may be caused to run too much in some cases, or not enough in other cases, thereby causing a flood.